Don't lock all current and future memory if can't increase memlock rlimit

If we fail to increase our RLIMIT_MEMLOCK, then locking all our current
and future memory is extremely dangerous; once our memory use reaches
our RLIMIT_MEMLOCK, memory allocations will start failing, very likely
leading to our entire process crashing.

This can happen if we aren't a privileged process, for example if
running as non-root user, or inside an unprivileged container.